Modems, or remote terminal units, convey information from one location to another over a data communications connection. Digital Subscriber Line (DSL) technology now enables modems to communicate large amounts of data. Modems normally communicate by modulating a baseband signal carrying digital data, precoding the transmit signal to match channel impulse response characteristics, converting the modulated digital data signal to an analog signal, and transmitting the analog signal over a conventional copper wire pair using techniques that are known in the art. These known techniques include mapping the information to be transmitted into a signal space constellation, differentially encoding the information, and modulating the information. The signal space constellation can include both analog and digital information or merely digital information.
Communication signals, such as those used in digital subscriber line (DSL) technology, that are transmitted over an analog copper wire pair must meet specific power and frequency requirements as required by government regulations. These requirements dictate that a limited amount of energy per unit of bandwidth be transmitted for a given portion of the frequency spectrum. Typically, higher power is permitted to be transmitted at lower frequencies, while as transmit frequency increases, the power transmitted must be decreased. These government regulatory requirements are designed to limit interference and are imposed on all services that share the analog communications system. Previously, there has been no way to provide the required spectral shaping without unduly limiting the available transmit power.
Furthermore, conventional preceding techniques as described in U.S. Pat. No. 5,396,515, U.S. Pat. No. 5,559,835 and U.S. Pat. No. 5,159,610, and specified in the International Telecommunications Union-Telecommunications (ITU-T) V0.34 standard require a receiver to reply to a transmitter with information regarding the channel response of the communications channel during a training period so as to enable the transmitter to set its precoder. This requires a lengthy training period in which a modem transmitter awaits information from a receiver in order to set the transmitter's precoder in order to maximize transmission efficiency.